1. Field of the Invention
This invention relates to thermally responsive crystals, more particularly to inverse opal photonic crystals containing material within the voids of the inverse opal that is responsive to temperature changes, thereby changing the band gap of radiation reflected by the photonic crystal.
2. Description of Related Art
Photonic crystals are optical materials in which the refractive index varies in multiple dimensions. Photonic crystals may be produced from crystalline colloidal arrays that reflect radiation according to Bragg's law over a range of wavelengths that depends on the composition of the materials in the array, the particle size, the packing arrangement within the array, and the degree of regularity of the array. Crystalline colloidal arrays have been used as three-dimensional ordered arrays of mono-dispersed colloidal particles that are often composed of a polymer latex such as polystyrene or an inorganic material such as silica. Colloidal dispersions of particles can form crystalline structures having lattice spacings that are comparable to the wavelength of radiation, within the ultraviolet, visible, or infrared wavelengths. Such crystalline structures have been used for filtering narrow bands from selected wavelengths from a broad spectrum of incident radiation, while permitting transmission of adjacent wavelengths of radiation.
Such crystalline colloidal arrays typically have a constant inter-particle spacing within the array, whereas other crystalline colloidal arrays may be thermally active when the inter-particle spacing varies in response to stimuli, such as temperature change. Thermally responsive crystalline colloidal arrays traditionally are produced from hydrogels. In hydrogel-based devices, mono-dispersed, highly-charged colloidal particles are dispersed in aqueous media. The particles self-assemble into a crystalline colloidal array due to the electrostatic charges. The ordered structure diffracts radiation according to Bragg's law, wherein the radiation meeting the Bragg condition is reflected while adjacent spectral regions that do not meet the Bragg conditions are transmitted through the device. An array of particles that diffract radiation according to Bragg's law satisfies the equation:mλ=2nd sin θwhere m is an integer, λ is the wavelength of reflected radiation, n is the effective refractive index of the array, d is the distance between the layers of particles, and θ is the angle that the reflected radiation makes with a plane of a layer of the particles. Thus, by increasing particle size or the volume of the matrix between layers of particles, the interparticle distance (d) between the layers of particles increases, thereby changing the wavelength of diffracted radiation. The particle size and/or the matrix volume may increase in response to a stimulus, such as a temperature change that causes the particles or the matrix to swell. Likewise, a change in the effective refractive index of the array can also shift the wavelength of diffracted radiation.
Other photonic crystals are based on inverse opals. Synthetic opal structures have been produced from uniformly-sized submicron silica spheres that are arranged into an ordered periodic array. The voids between the silica spheres are filled with a matrix material followed by dissolution of the silica spheres to yield a periodic array of voids within the uniform matrix material. The voids may be filled with a filler composition in order to adjust the optical properties of the inverse opal.